1. Field of the Invention
The present invention pertains to improvements in magnetic field sensing compasses utilizing magnetic azimuth or flux valve pickoffs in which sensed mutually perpendicular components of the earth's field are nulled by the closed loop feed back of nulling currents into respective pairs of flux valve sensing windings.
2. Description of the Prior Art
A known flux valve earth's field directional pickoff is a structure having a cooperating equiangular Y-shaped laminated core, an excitation winding, and sensing windings surrounding and coupled to each of the three legs of the core and disclosed widely in the literature, as in the O. E. Esval, C. F. Fragola, and L. F. Beach U.S. Pat. No. 2,383,461 for a "Flux Valve Compass System," issued Aug. 28, 1945, and in the M. C. Depp U.S. Pat. No. 2,852,859 for "Flux Valve Compensating System," issued Sept. 23, 1958, both parents being assigned to Sperry Rand Corporation. Such flux valves are normally mounted with the plane of the core disposed horizontally so that they measure the earth's total horizontal magnetic field, if properly compensated for any error effects produced by spurious magnetic fields associated with the structure of the craft on which the flux valve is mounted. Such flux valves are directly suited to use in open loop three-wire selsyn data transmission systems either for slaving a directional gyroscope, or for providing a direct analog representation or indication of the angle of a craft longitudinal axis relative to magnetic north at a receiving device. However, such open-loop systems suffer various disadvantages. For example, the output of the Y-shaped flux valve is rich in harmonics and accurate demodulation is difficult, especially for low signal levels. Craft vibrations also produce spurious flux valve outputs. Conversion of the alternating currents from the flux valve into unidirectional sine and cosine signals of sufficient accuracy as often required in navigation equipment is difficult.
Closed loop compass systems were then realized to overcome some of the foregoing difficulties of the prior art, such as the compass systems of the D. H. Baker, F. H. Kallio U.S. Pat. No. 3,678,593 for a "Compass System and Components Thereof Having Automatic Field Compensation," issued July 25, 1972, and of the J. R. Erspamer U.S. Pat. No. 3,942,257 for "Index Error Correction for Flux Valve Heading Repeater System," issued Mar. 9, 1976, both patents being assigned to Sperry Rand Corporation. In these systems, the output of a three-legged flux valve is converted into direct current signals representative of the components of the sensed earth's field vector. These currents provide the useful compass output signals and are also fed back into the legs of the flux valve in a direction which substantially nulls the earth's field vector. Thus, the compass operates about a null condition in a closed loop manner affording improved directional accuracy particularly as discussed in the aforementioned Baker et al patent.